One thing I noticed is that the pops seem to always start with the incoming IM noise emitted by Skype. Another thing I noticed was that the pop seemed to be first coming from my 24” Cinematic Display. To simplify the number of things going on, I disabled the Cinematic Display (aka "Apple USB audio device”) and “Digital Audio (S/PDIF)” playback audio devices in mmsys.cpl because I’m not using them. (I’ve gone many hours without any pops and crackles after disabling these two devices but I’m not ready to say the problem is solved.)

What I just noticed is that the incoming IM ping sound was coming very much from my right speaker and it was really loud. Then I realized it wasn’t coming from the right desktop speaker at all. It was coming from my laptop. All of my other sound is coming out of my Klipsch speakers via the “headphones” minijack. Nothing should be coming out of the laptop speakers which aren’t the current default playback device. It shouldn’t be doing that.

The Volume Mixer shows that Skype is the only thing using the built-in speakers. These screenshots were taken nearly simultaneously without making any system changes. I just changed the selected device in the Volume Mixer.

All of the Skype incidental notifications are coming out of my laptop but voice calls are playing out of the configured default playback device, which is my desktop speakers. Very weird. This reminds me that when we first got the MacBooks it was just as Snow Leopard was released and we had the drivers in Boot Camp 3.0. Skype could not hear anything with the built-in microphone even though the built-in microphone was working as demonstrated by Sound Recorder.

I have a suspicion that Skype is doing something low-level and inadvisable with the audio devices.

For the record, according to Skype’s own settings dialogs, it should not be using the laptop built-in speakers.

Mystery Solved!

The notifications show up on whatever speaker is defined in Options | Audio Settings | Ringing. By default that is set to “Ring on all devices”. Changing it to Use selected speaker removes the weirdness of ringing coming out of the laptop speakers while using headphones.